A shift in transitional forests of the North American boreal will persist through 2100

High northern latitude changes with Arctic amplification across a latitudinal forest gradient suggest a shift towards an increased presence of trees and shrubs. The persistence of change may depend on the future scenarios of climate and on the current state, and site history, of forest structure. Here, we explore the persistence of a gradient-based shift in the boreal by connecting current forest patterns to recent tree cover trends and future modeled estimates of canopy height through 2100. Results show variation in the predicted potential height changes across the structural gradient from the boreal forest through the taiga-tundra ecotone. Positive potential changes in height are concentrated in transitional forests, where recent positive changes in cover prevail, while potential change in boreal forest is highly variable. Results are consistent across climate scenarios, revealing a persistent biome shift through 2100 in North America concentrated in transitional landscapes regardless of climate scenario.

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REVIEWER COMMENTS:
Reviewer #1 (Remarks to the Author): COMMSENV-24-0402-T Review report In this manuscript entitled "Persistence of a boreal forest shift in North America through 2100: transitional landscapes feature recent and predicted growth in vegetation structure regardless of climate scenario," the authors present an analysis of potential changes in tree and shrub cover through the end of the century.They propose a somewhat new modelling method in which the current gradient in canopy height is modelled as a function of 22 climate predictor variables.The model is then used to project future changes in canopy height with future climate predictions with a classic space-for-time approach.Such analyses are needed, given the important changes in forest ecosystems that have been observed in the last decades.Overall, the analyses conducted were sound and well-explained, and the whole manuscript was well-written and pleasant to read.For all these reasons, I think this manuscript is worth publishing in the Communications Earth and Environment.However, I have a few main comments that I'll develop below.
Although I like the effort to predict future changes with the space-for-time approach, it feels somewhat curious to present the predicted future trends before the observed recent trends.I understand that the model results presented are not based on the data presented in Figure 4 because the ICESat-2 lidar data are quite new and do not allow trend reconstructions in canopy height.However, I still suggest that the observed recent trends (Figure 4) should be presented before the projections of future changes.
My second concern is about the fact that historical factors are not considered in the model.Forest height is only modelled as a function of climatic variables, while historical factors such as fire are integral to the natural ecosystem dynamics (Gauthier et al. 2015).Some authors have developed similar approaches (though to a smaller extent) to model canopy height growth and accounting disturbance history (Pau et al. 2021, Danneyrolles et al. 2023).I realize that it would be rather complicated (though not impossible) to include the effect of fire in the model on this continental scale.I also note that the authors acknowledge in their last discussion section that "Site factors and stochastic processes will modify predictions."I suggest the authors elaborate further and more precisely on this aspect of the discussion.More particularly, they should discuss how the relation of vegetation height with climate will be non-linear due to fire feedback.Climate change is increasing fire in many parts of the North American boreal forests (Jain et al. 2017, Whitman et al. 2022, Barnes et al. 2023).Fire feedback includes short-to medium-term reductions in vegetation height, species change (i.e., changes in maximum potential height), and regeneration failure (Baltzer et al. 2021).Another aspect to discuss is that if tree or shrub vegetation moves north into the tundra, as predicted by the authors, the fire will also move north following the increase in fuel.The study covers an important topic of interest to a large audience-spatial shifts in the North American boreal forest with climate change.This article specifically finds that boreal vegetation will expand into the tundra under a variety of future climate scenarios.This is indicated by projected increases in heights of vegetation with future projected warmer climates, in places where there are currently only scattered trees of short stature.Also, there will be variable changes in potential height within the current main range of the boreal biome, in some places trees are projected to become shorter as climate and site become unfavorable, while in other places tree heights will increase.
These findings are supported by the data and nicely illustrated in the figures.I think mapping current and potential future tree height is the best way to project the future condition of the boreal forest, given the recent availability of lidar data over large areas, and the historic usefulness of tree height as an indicator of forest condition.I consider the findings to be novel because previously there were individual site studies in the field showing that trees are taking advantage of recent warming, or biome level analyses showing projected northward expansion of the boreal biome.However, this paper uses 30 m pixel data nested within 1 km grid squares, combined with models of tree response to climate projected into the future, to show how the whole North American boreal biome will change-in other words it has the maximum combination of spatial detail and large geographical extent that can be done at this time.
The data assembly processing and main conclusions are very good, based on the best available and most recent remotely sensed tree heights and data on site quality.I have used similar statistical methods in some of my published papers (e.g.random forests, regression trees using the bioclimatic variables), and it looks to me like the analyses here were done properly.
The Discussion also makes good points about the significant findings of the study and puts them in the context of previous literature.It also appropriately points out the limitations of the study with respect to disturbances and other factors that are not mapped that likely influence the current and future height of boreal forests.However, using maximum potential height minimizes these influences by choosing grid cells with the 98th percentile in heights within a given 1 km grid square, since it is well known that even in recently burned areas, there are remnant stands of trees that represent to potential height of the forest.I do recommend some minor revisions to make the paper suitable for publication: The second sentence of the introduction is confusing.It makes it sound as if the entire boreal biome is experiencing an increase in presence of trees and shrubs, whereas it is the transition zone to tundra along the northern edge of the boreal biome that is experiencing increases in trees and shrubs.
The Discussion does not mention that along the southern ecotone of the boreal forest, there may be transitions to other biome types that exist just south of the boreal: grasslands (in central continental areas-the 'prairie provinces' of Canada) or to temperate forests (from eastern Minnesota across eastern Canada to the Atlantic Ocean).At a minimum, the Discussion should mention that these transitions were outside the scope of this study, but nevertheless are likely to occur, and perhaps that we don't know the extent to which they will negate expansion in the north (i.e.whether the boreal as a whole will get smaller or larger).
With regard to the methods section in the supplementary material, everything in this paper hinges on modeling potential canopy height, and the explanation of this process needs to be totally clear.Am I correct in surmising that there were ca 1,100 30 x 30 m pixels in each square km, and that ca 22 of those 1,100 pixels with the tallest lidar-based tree height were averaged together and then used to represent each square km in the regression model?If so, I would state this explicitly either in/near lines 72-78 or lines 89-91 of the methods.Or if I am wrong state it correctly, or even give a formula if possible.
Finally, also in the supplementary/methods section, I noticed that Newfoundland and part of Nova Scotia are not included on the maps, whereas they are included on maps in the main article.A sentence or two about this discrepancy should probably be included in the methods, or else provide revised maps in the supplementary material.

Title
Persistence of a North American boreal forest shift through 2100: transitional landscapes feature recent and predicted growth in vegetation structure

Response to Referees
The Reviewers have provided a thorough critique of the manuscript and have shared a number of thoughtful comments (italics) and have provided responses to all of them (bold).We have carefully considered each comment and have, for nearly all comments, updated the manuscript in the manner that was suggested.We have otherwise provided justification for maintaining what we have.During this process, we have added a co-author who helped with dataset development and manuscript revision.The important updates made at the Reviewers' requests include: 1. Updated text in the Discussion on the importance of historical factors on potential forest height and to note we do not examine in detail the southern edge of the boreal.2. Clarification of Methods on gridding of ICESat-2 3.An update to Supplementary Figure 2 to correct the omission of a small portion of the prediction and analysis domain in eastern Canada, and a corresponding update to the associated text.
The Reviewers' comments and the updates based on them have made this manuscript clearer and stronger, and we thank them for the consideration they have given to our work. Sincerely,

Paul Montesano
Reviewer #1 (Remarks to the Author): In this manuscript entitled "Persistence of a boreal forest shift in North America through 2100: transitional landscapes feature recent and predicted growth in vegetation structure regardless of climate scenario," the authors present an analysis of potential changes in tree and shrub cover through the end of the century.They propose a somewhat new modelling method in which the current gradient in canopy height is modelled as a function of 22 climate predictor variables.The model is then used to project future changes in canopy height with future climate predictions with a classic space-for-time approach.Such analyses are needed, given the important changes in forest ecosystems that have been observed in the last decades.Overall, the analyses conducted were sound and well-explained, and the whole manuscript was well-written and pleasant to read.For all these reasons, I think this manuscript is worth publishing in the Communications Earth and Environment.However, I have a few main comments that I'll develop below.
Although I like the effort to predict future changes with the space-for-time approach, it feels somewhat curious to present the predicted future trends before the observed recent trends.I understand that the model results presented are not based on the data presented in Figure 4 because the ICESat-2 lidar data are quite new and do not allow trend reconstructions in canopy height.However, I still suggest that the observed recent trends (Figure 4) should be presented before the projections of future changes.

RESPONSE:
The Reviewer's comment is understandable in that presenting the forest structure data in a chronological order is in some ways quite intuitive.Here, the focus is on predictions of potential future change, the predictive power of current structure, and the historical context that recent trends impart on these predictions.In large part because of our choice to drive our predictions of potential boreal forest change using various emissions scenarios, and then contextualize them with current spatial patterns of structure and recent temporal trends of structure.The authors assert that bringing in this historical context after the presentation of predictions and their variation across CMIP scenarios and geographic space, provide the most compelling explanation of the patterns in the predictions.
My second concern is about the fact that historical factors are not considered in the model.Forest height is only modelled as a function of climatic variables, while historical factors such as fire are integral to the natural ecosystem dynamics (Gauthier et al. 2015).Some authors have developed similar approaches (though to a smaller extent) to model canopy height growth and accounting disturbance history (Pau et al. 2021, Danneyrolles et al. 2023).I realize that it would be rather complicated (though not impossible) to include the effect of fire in the model on this continental scale.I also note that the authors acknowledge in their last discussion section that "Site factors and stochastic processes will modify predictions."I suggest the authors elaborate further and more precisely on this aspect of the discussion.More particularly, they should discuss how the relation of vegetation height with climate will be non-linear due to fire feedback.Climate change is increasing fire in many parts of the North American boreal forests (Jain et al. 2017, Whitman et al. 2022, Barnes et al. 2023).Fire feedback includes short-to medium-term reductions in vegetation height, species change (i.e., changes in maximum potential height), and regeneration failure (Baltzer et al. 2021).Another aspect to discuss is that if tree or shrub vegetation moves north into the tundra, as predicted by the authors, the fire will also move north following the increase in fuel.

RESPONSE:
The Reviewer highlights the importance of historical factors and wildfire characteristics (frequency, severity, site responses, etc) in the prediction of future vegetation structure.In this work, such historical factors (eg., previous wildfire severity) are in some way accounted for insofar as they contribute to the expression of the current vegetation height that is captured with ICESat-2 data.However, this type of contribution of site history to predictions of vegetation height is coarse and indirect.We have elaborated on this missing control of height in our predictions at the Reviewer's request in the final paragraph of the Discussion.We point out how site-level fire dynamics may alter predictions at the pixel-level significantly.Furthermore, we point out that other regional variability that we don't account for may continue to control patterns of vegetation structure change.
Reviewer #2 (Remarks to the Author): The study covers an important topic of interest to a large audience-spatial shifts in the North American boreal forest with climate change.This article specifically finds that boreal vegetation will expand into the tundra under a variety of future climate scenarios.This is indicated by projected increases in heights of vegetation with future projected warmer climates, in places where there are currently only scattered trees of short stature.Also, there will be variable changes in potential height within the current main range of the boreal biome, in some places trees are projected to become shorter as climate and site become unfavorable, while in other places tree heights will increase.
These findings are supported by the data and nicely illustrated in the figures.I think mapping current and potential future tree height is the best way to project the future condition of the boreal forest, given the recent availability of lidar data over large areas, and the historic usefulness of tree height as an indicator of forest condition.
I consider the findings to be novel because previously there were individual site studies in the field showing that trees are taking advantage of recent warming, or biome level analyses showing projected northward expansion of the boreal biome.However, this paper uses 30 m pixel data nested within 1 km grid squares, combined with models of tree response to climate projected into the future, to show how the whole North American boreal biome will change-in other words it has the maximum combination of spatial detail and large geographical extent that can be done at this time.
The data assembly processing and main conclusions are very good, based on the best available and most recent remotely sensed tree heights and data on site quality.I have used similar statistical methods in some of my published papers (e.g.random forests, regression trees using the bioclimatic variables), and it looks to me like the analyses here were done properly.
The Discussion also makes good points about the significant findings of the study and puts them in the context of previous literature.It also appropriately points out the limitations of the study with respect to disturbances and other factors that are not mapped that likely influence the current and future height of boreal forests.However, using maximum potential height minimizes these influences by choosing grid cells with the 98th percentile in heights within a given 1 km grid square, since it is well known that even in recently burned areas, there are remnant stands of trees that represent to potential height of the forest.
I do recommend some minor revisions to make the paper suitable for publication: The second sentence of the introduction is confusing.It makes it sound as if the entire boreal biome is experiencing an increase in presence of trees and shrubs, whereas it is the transition zone to tundra along the northern edge of the boreal biome that is experiencing increases in trees and shrubs.

RESPONSE:
We agree with the Reviewer and have adjusted the wording accordingly.
The Discussion does not mention that along the southern ecotone of the boreal forest, there may be transitions to other biome types that exist just south of the boreal: grasslands (in central continental areas-the 'prairie provinces' of Canada) or to temperate forests (from eastern Minnesota across eastern Canada to the Atlantic Ocean).At a minimum, the Discussion should mention that these transitions were outside the scope of this study, but nevertheless are likely to occur, and perhaps that we don't know the extent to which they will negate expansion in the north (i.e.whether the boreal as a whole will get smaller or larger).

RESPONSE:
The Reviewer makes a good point that it may be important to be more explicit in that we are focusing our analysis of potential change in the transitional forests of the boreal-tundra, and not the southern limit of the boreal.We have updated the end of the 2nd paragraph of the Discussion accordingly.
With regard to the methods section in the supplementary material, everything in this paper hinges on modeling potential canopy height, and the explanation of this process needs to be totally clear.Am I correct in surmising that there were ca 1,100 30 x 30 m pixels in each square km, and that ca 22 of those 1,100 pixels with the tallest lidar-based tree height were averaged together and then used to represent each square km in the regression model?If so, I would state this explicitly either in/near lines 72-78 or lines 89-91 of the methods.Or if I am wrong state it correctly, or even give a formula if possible.